1. Technical Field of the Invention
Isotopic analyses are useful for diagnosis of a disease in a medical application, in which metabolic functions of a living body can be determined by measuring a change in the concentration or concentration ratio of an isotope after administration of a drug containing the isotope. In the other fields, the isotopic analyses are used for studies on the photosynthesis and metabolism of plants, and for ecological tracing in a geochemical application.
The present invention relates to stable isotope measurement methods and apparatus for spectrometrically measuring the concentration or concentration ratio of an isotopic gas on the basis of the light absorption characteristics of the isotope.
2. Background Art
It is generally known that gastric ulcer and gastritis are caused by bacteria called helicobacter pylori (HP) as well as by a stress.
If the HP is present in the stomach of a patient, an antibiotic or the like should be administered to the patient for bacteria removal treatment. Therefore, it is indispensable to check if the patient has the HP. The HP has a strong urease activity for decomposing urea into carbon dioxide and ammonia.
Carbon has isotopes having mass numbers of 12, 13 and 14, among which 13C having a mass number of 13 is easy to handle because of its non-radioactivity and stability.
If the concentration of 13CO2 as a final metabolic product or the concentration ratio of 13CO2 to 12CO2 in breath of a patient is successfully measured after urea labeled with the isotope 13C is administered to the patient, the presence of the HP can be confirmed.
However, the concentration ratio of 13CO2 to 12CO2 in naturally occurring carbon dioxide is 1:100. Therefore, it is difficult to determine the concentration ratio in the breath of the patient with high accuracy.
There have been known methods for determining the concentration ratio of 13CO2 to 12CO2 by way of infrared spectroscopy (see Japanese Examined Patent Publications No. 61-42219 (1986) and No. 61-42220 (1986)).
In the method disclosed in Japanese Examined Patent Publication No. 61-42220, two cells respectively having a long path and a short path are provided, the path lengths of which are adjusted such that the light absorption by 13CO2 in one cell is equal to the light absorption by 12CO2 in the other cell. Light beams transmitted through the two cells are led to the detectors, in which the light intensities are measured at wavelengths which ensure the maximum sensitivity. In accordance with this method, the light absorption ratio for the concentration ratio of 13CO2 to 12CO2 in naturally occurring carbon dioxide can be adjusted to 1. If the concentration ratio is changed, the light absorption ratio also changes by the amount of a change in the concentration ratio. Thus, the change in the concentration ratio can be determined by measuring the change in the light absorption ratio.
A. However, the method for determining the concentration ratio according to the aforesaid literature suffers from the following drawback.
Calibration curves for determining the concentrations of 12CO2 and 13CO2 should be prepared by using gas samples each having a known 12CO2 concentration and gas samples each having a known 13CO2 concentration.
To prepare the calibration curve for the 12CO2 concentration, the 12CO2 absorbances are measured for different 12CO2 concentrations. The 12CO2 concentrations and the 12CO2 absorbances are plotted as abscissa and ordinate, respectively, and the calibration curve is determined by the method of least squares.
The calibration curve for the 13CO2 concentration is prepared in the same manner as described above.
The 13CO2 concentration or the 13CO2 concentration ratio (which is herein meant by 13CO2 concentration/12CO2 concentration) in the breath as a test gas sample is typically determined by way of infrared spectroscopy. In this case, since a test sample gas, or breath is exhaled from a living body as a result of the metabolism, the breath contains water vapor in a concentration proximate to saturation.
In the infrared spectroscopy, the absorption of infrared radiation with a particular wavelength by a test gas sample is utilized for determination of the absorbance for the test gas sample.
FIG. 5 is a graph obtained by plotting the measured values of the 13CO2 concentration ratio changes with respect to the humidities of test gas samples having different humidities ranging from 0% to 100% wherein the 13CO2 concentration ratio with respect to a 0%-humidity gas sample is used as a reference gas sample.
As can be seen from the graph, the measured values of the 13CO2 concentration ratio are not the same, but vary depending on the humidity.
Therefore, if the 13CO2 concentration or the 13CO2 concentration ratio of a test gas sample containing moisture is measured in ignorance of this fact, the measured value is apparently greater than the true value.
One approach to this problem is to remove the moisture contained in the breath sample as the test gas sample through molecular sieving or with the use of a moisture absorbent such as magnesium perchlorate prior to the measurement. However, some problems may be encountered in this approach since the approach requires a large space for housing the moisture absorbent, there is no means for checking if the moisture is completely removed by the moisture absorbent, and the moisture absorbent should periodically be replaced with a new one.
It is, therefore, an object of the present invention to provide a stable isotope measurement method and apparatus for spectrometrically analyzing an isotopic gas, wherein a test gas sample containing carbon dioxide 13CO2 as a component gas is introduced into a cell and the concentration or concentration ratio of the component gas is precisely measured and corrected by measuring moisture content in the test gas sample.
A stable isotope measurement method for spectrometrically analyzing an isotopic gas in accordance with the present invention comprises: a first step of introducing a test gas sample into a cell and determining the absorbance of light transmitted therethrough at a wavelength suitable for the component gas 13CO2; a second step of determining a concentration of the component gas in the test gas sample on the basis of a calibration curve prepared through measurement on test gas samples each containing the component gas in a known concentration; and a third step of measuring a concentration of water vapor contained in the test gas sample and correcting a concentration of the component gas contained in the test gas sample in accordance with the measured water vapor concentration on the basis of a correction curve prepared through measurement on test gas samples each containing water vapor in a known concentration.
A stable isotope measurement method for spectrometrically analyzing an isotopic gas in accordance with the present invention comprises: a first step of introducing a test gas sample containing carbon dioxide 13CO2 and carbon dioxide 13CO2 as component gases into a cell and determining the absorbances of light transmitted therethrough at wavelengths suitable for the respective component gases; a second step of determining a concentration ratio between the component gases in the test gas sample on the basis of a calibration curve prepared through measurement on test gas samples each containing the component gases in known concentrations; and a third step of measuring a concentration of water vapor contained in the test gas sample and correcting a concentration ratio between the component gases contained in the test gas sample in accordance with the measured water vapor concentration on the basis of a correction curve prepared through measurement on test gas samples each containing water vapor in a known concentration.
When compared with the prior art method, each of the aforesaid methods additionally include the third step in which the concentration ratio of the component gas is corrected in accordance with the measured water vapor concentration on the basis of the correction curve prepared through the measurement on the test gas samples each containing water vapor in a known concentration.
Although the concentration of the component gas should basically be represented by a single true value, the measured value of the concentration of the component gas varies depending on the water vapor concentration. In view of this fact, the aforesaid methods improve the measurement accuracy of the concentration ratio of the component gas.
The water vapor concentration may otherwise be determined by means of any of various humidity sensors, or may be calculated from the absorbance determined spectrometrically on the basis of the water molecule spectrum.
In the method of claim 2, the correction curve in the third step is prepared by determining the light absorbances at the wavelengths suitable for the respective component gases for the plurality of test gas samples containing water vapor in different concentrations, then determining the concentrations of or concentration ratios between the respective component gases in the test gas samples on the basis of the calibration curve, and plotting ratios or differences between the concentrations of or the concentration ratios between the respective component gases in the gas samples thus determined with respect to the water vapor concentrations, and the correction in the third step is achieved by obtaining a concentration correction value or a concentration ratio correction value for the component gases by fitting the water vapor concentration of the test sample gas obtained in the third step to the correction curve, and then dividing the concentrations of or the concentration ratio between the respective component gases in the test gas sample obtained in the second step by the concentration correction value or the concentration ratio correction value obtained on the basis of the correction curve, or subtracting the concentration correction value or the concentration ratio correction value from the concentrations of or the concentration ratio between the respective component gases in the test gas sample.
A stable isotope measurement apparatus for spectrometrically analyzing an isotopic gas in accordance with the present invention is a measurement apparatus adapted to perform the aforesaid methods for spectrometrically analyzing the isotopic gas and comprises, as data processing means, absorbance calculation means for determining the absorbances of light transmitted through the test gas sample introduced into the cell on the basis of light intensities measured at the wavelengths suitable for the respective component gases, concentration calculation means for determining the concentration ratio of the component gases on the basis of the calibration curve prepared through the measurement on the test gas samples each containing the component gases in known concentrations, water vapor concentration measuring means for measuring the concentration of water vapor contained in the test gas sample, and correction means for correcting the concentration ratio between the component gases in the test gas sample in accordance with the, measured water vapor concentration on the basis of the correction curve prepared through the measurement on the gas samples each containing water vapor in a known concentration.
In the methods or apparatus for spectrometrically analyzing the isotopic gas in accordance with the present invention, when a test gas sample containing carbon dioxide 13CO2 as a component gas is introduced into the cell and then spectrometrically analyzed, the concentration ratio of the component gas is corrected in accordance with the water vapor concentration in the test gas sample. Therefore, the concentration ratio of the component gas can be determined with a higher accuracy.
B. In the infrared spectrometric analysis, the 12CO2 concentration in a breath sample obtained before the drug administration is calculated from the measured 12CO2 absorbance on the basis of a 12CO2 calibration curve, while the 13CO2 concentration in the breath sample is calculated from the measured 13CO2 absorbance on the basis of a 13CO2 calibration curve. The 12CO2 and 13CO2 concentrations in the breath sample obtained after the drug administration are determined in the same manner.
If the CO2 concentrations in the two breath samples are substantially the same, it is possible to use narrower ranges of the 12CO2 calibration curve and the 13CO2 calibration curve. Thus, the measurement accuracy can be improved by using limited ranges of the calibration curves.
For equalization of the CO2 concentrations in the two breath samples, either one of the breath samples should be diluted. Typically used as a gas for dilution (hereinafter referred to as xe2x80x9cdiluent gasxe2x80x9d) is nitrogen gas which exhibits no absorption in the infrared region of the radiation spectrum (nitrogen gas is used as the diluent gas in the embodiment of the invention disclosed in Japanese Unexamined Patent Publication No. 9-1665546 (1997) which was filed prior to the present invention).
In this dilution method, however, the diluted breath sample has a different component gas ratio from the undiluted breath sample, because diluent gas contains only nitrogen but breath sample contains oxygen, moisture and etc. as well as nitrogen.
As a result, the difference in the component gas ratio influences the determination of the 13CO2 concentration and the concentration ratios between 12CO2 and 13CO2, so that the measured values may be erroneous.
It is, therefore, another object of the present invention to provide a method for spectrometrically analyzing an isotopic gas, wherein a breath sample as a test gas sample containing a plurality of component gases is introduced into a cell and the concentrations of the component gases are precisely measured through spectrometry by diluting the test gas sample in such a manner that the component gas composition in the test gas sample is not changed.
To achieve this object, there is provided a stable isotope measurement method for spectrometrically analyzing an isotopic gas, wherein two test gas samples are sampled from a single subject and, if the CO2 concentration of one of the test gas samples is higher than the CO2 concentration of the other test gas sample, the one test gas sample is diluted with air (atmospheric air) to a CO2 concentration level which is equivalent to that of the other test gas sample for measurement of the concentration ratios 13CO2/12CO2 in the respective test gas samples (claim 5).
In this method, the two breath samples are analyzed on condition that the breath samples have the same CO2 concentration level. This makes it possible to use limited ranges of the calibration curves. In addition, the component gas composition in the breath sample is not changed by the dilution because air is used as the diluent gas. As a result, the measurement accuracy can be improved.
Methods according to present claims 6 and 7 each provide a more specific procedure for the method for spectrometrically analyzing the isotopic gas in accordance with claim 5, and are each based on the precondition that a first test gas sample is first filled in a single cell for measurement of the intensity of light transmitted therethrough and, after the first test gas sample is discharged from the cell, a second test gas sample is filled in the cell for measurement of the intensity of light transmitted therethrough.
As described above, the CO2 concentrations in the two test gas samples can be generally equalized by diluting either one of the two test gas samples so as not to change the component gas composition of the breath sample. This makes it possible to use limited ranges of the 12CO2 and 13CO2 calibration curves. The accuracy of the calibration curves is increased as the ranges of the calibration curves to be used are narrowed. Therefore, the measurement accuracy can be improved by limiting the ranges of the calibration curves to be used.